Most of the plumbing is already present in the non-public API. According to
ES2016, Symbols are also accepted in calls to getOwnProperty(), and taking
them is required in Blink for proper record<K,V> WebIDL conversions.

R=jochen@chromium.org,verwaest@chromium.org

Bug: chromium:724481
Change-Id: I0dfe0e57f6d811f04ecbfd8ec0c97e44c9f02c96
Reviewed-on: https://chromium-review.googlesource.com/509611Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Raphael Kubo da Costa (rakuco) <raphael.kubo.da.costa@intel.com>
Cr-Commit-Position: refs/heads/master@{#45454}